1. Field of the Invention
The present invention relates to a magnetic recording medium. Specifically, the present invention relates to a coating-type high-recording-density magnetic recording medium having excellent surface properties and exhibiting excellent performances in a total recording, playback system used in short wavelength regions, and in particular, having an electromagnetic transducing characteristic satisfactorily adaptable to a high-recording-density medium system using an MR head and the like, as well as excellent reliability.
2. Description of the Related Art
In recent years, recording densities of magnetic recording media have been increased, and therefore, further improvements of the electromagnetic transducing characteristic, e.g. reduction in wavelength (for example, a shortest recording wavelength specified in DDS-4, which is one standard on data tapes, is in the order of 0.3 μm), shift toward a digital recording system and shift toward a system equipped with an MR head for playback (the shortest recording wavelength is 0.25 μm or less), are required with respect to magnetic recording media. In order to satisfy these requirements, reduction in the magnetic layer thickness is under consideration with respect to the coating-type magnetic recording media.
By reason of the above-mentioned requirements, regarding magnetic recording media including a magnetic layer which contains a ferromagnetic alloy powder (hereafter may be referred to simply as “a magnetic powder” or “a powder”) and a binder and which has a reduced layer thickness, so-called multilayer structures provided with a non-magnetic layer between a non-magnetic support and a magnetic layer are often adopted as one means of avoiding problems, such as reduction in an output, noises and head touches during recording and playback, resulting from a surface condition on the non-magnetic support up to this time. In this manner, the electromagnetic characteristic of the magnetic layer having a reduced layer thickness have been also improved. However, even the magnetic recording medium having the multilayer structure has been insufficient to achieve a surface recording density of 0.5 Gbit/inch2 or more, in particular 1 Gbit/inch2 or more.
In order to increase the ratio of a carrier output to a noise output (C/N or S/N) on a unit volume (unit area) basis for increasing a density, reduction in noises, especially noises due to particles, is required, and therefore, reduction in the size of a magnetic powder is attempted. Wh n the size of the magnetic powder is reduced, the number of the particles on a unit volume basis is increased, and the particulate noises can be reduced. Theory holds that the above-mentioned effect can be achieved because when the number of the particles increases by a factor of n, the carrier output increases by a factor of n, and the noise output increases by a factor of the square root of n. Therefore, theoretically, the carrier output/noise output ratio increases by a factor of the square root of n. However, uniform dispersion became difficult as the magnetic powder became small, and when a conventional magnetic powder was used, it was difficult to achieve the above-mentioned theoretical effect through reduction in the size of the magnetic powder.
On the other hand, Japanese Unexamined Patent Application Publication No. 9-91697 discloses that a ferromagnetic alloy powder having an average major-axis length of 0.08 μm or less is used, an ultrasonic treatment is applied to a magnetic paint, and thereby dispersibility is increased, the surface properties and magnetic characteristics of the magnetic layer, in particular the squareness ratio (residual magnetic flux density/saturation magnetic flux density) and the degree of orientation in a plane, are improved, and a magnetic recording medium exhibiting a high electromagnetic transducing charact ristic is produced. Japanese Unexamined Patent Application Publication No. 11-251122 and Japanese Unexamined Patent Application Publication No. 2001-81506 disclose ferromagnetic metal powders produced by gas reduction of a powder composed of needle particles (average major-axis length: 0.01 to 0.40 μm), in which iron oxyhydroxide or iron oxide is allowed to contain Co, Al and Y under a predetermined condition, as ferromagnetic metal powders used for coating-type magnetic recording media suitable for high-recording-density recording.
The above-mentioned Japanese Unexamined Patent Application Publication No. 9-91697 discloses a medium having a squareness ratio of 0.88 or more in the plane of a magnetic layer and the degree of orientation of 3.0 or more with respect to a ferromagnetic alloy powder having an average major-axis length of 0.10 μm or more in an embodiment thereof. However, regarding the ferromagnetic alloy powder having an average major-axis length of less than 0.10 μm, an upper limit of the squareness ratio was 0.88 in the plane of the magnetic layer, and the upper limit of the degree of orientation was 2.8 from the result of calculation based on the data. That is, the use of the ferromagnetic alloy powder having an average major-axis length of less than 0.10 μm was insufficient to achieve further improvement in the electromagnetic transducing characteristic. Regarding both magnetic tapes described in the above-mentioned Japanese Unexamined Patent Application Publication No. 11-251122 and Japanese Unexamined Patent Application Publication No. 2001-81506, target thicknesses of the magnetic layers were 3 μm in the embodiments, and these thicknesses and the ferromagnetic metal powders applied to these thicknesses were insufficient to achieve further improvement in the electromagnetic transducing characteristics as well.